This invention relates to improvements in multicarrier demodulators of a type which are particularly useful in telecommunication systems employing satellite transmissions.
Multicarrier demodulators intended for use in satellite communication systems of the kind shown in FIG. 1 are well known. Referring to FIG. 1, various vehicles including aircraft 10, ships 11 and trucks 12 are interconnected with each other for radio communication via a satellite 13 and ground station 15. The satellite 13 receives uplink signals from one or more of the vehicles in the L band of frequencies, using a frequency division multiplexing mode (FDM), that includes up to 800 different channels. The satellite retransmits such data signals to the ground station 15 over the C band of frequencies using a time division multiplex (TDM) mode of transmission. From the ground station 15, the data is retransmitted to other ones of the mobile vehicles or stationary receivers.
The multicarrier demodulator of the present invention is employed as part of the transponder system aboard the satellite, as shown in block diagram form in FIG. 2. Referring to FIG. 2, the FDM uplink signal is received by the satellite antenna 18, is converted to a signal S.sub.FDM and passes to a first carrier frequency converter 19 having a reference signal generator 19a connected thereto. The converter signal S.sub.IF is passed through a filtering circuit 20 to a sampling circuit 21 and then to an analog-to-digital converter circuit 22, where the signal is digitalized as known in the art. After being digitalized, the signal is then passed to a frequency division demultiplexer 23, also known in the art, where the signal is separated into the digitalized signals for each of the many different channels 1 to L, inclusive. The digital separated signals for each channel are then demodulated by demodulator circuits 24a, 24b . . . 24n for each channel. In the satellite transponder, all of the different data signals from the different channels are combined by a multiplexer 25 and processor 27 into a time division multiplexer mode, modulated at 28 and transmitted in the different multiplex mode and at the different frequency band, by the satellite transmitting antenna 29.
The requirements for the processing and demodulating of the different signals for the channels 1 to L are as follows. A high sampling rate of f.sub.sI =4LB at sampling circuit 21 is required in relation to the number of channels L and the bandwidth B. This is reduced to a sampling frequency of f.sub.so =2B for the individual output signals for the plural L channels. The desired signal spectrum at the output of the demultiplexer 23 should be present in a centered position around a center band frequency f.sub.m which is not equal to zero and has a bandwidth B. It does not matter if outside undesirable spectral frequency components are present that could interfere with the demodulation since they are filtered to suppress the undesired frequencies. For demodulation, the center band frequency f.sub.m must be zero, and circuitry is provided to shift this frequency to such zero center position.
The data stream to be recovered in the demodulator is clocked with a stepping clock pulse having a frequency f.sub.s, with the sampling rate f.sub.so generally not being an integer multiple of f.sub.s. Since however, the data must be transferred at the output of the different channel demodulators with the stepping clock pulse frequency, the output pulse sampling rate f.sub.so must be adapted at a suitable location to the stepping clock pulse. For this purpose, an interpolation filter is required. This filter is also necessary if the output sampling rate is f.sub.so =m.multidot.fs, with m being a whole number, since the optimum sampling instant must be found in the demodulator with the aid of a clock pulse control loop to enable a subsequent decider to recover the data. This generally requires that further intermediate values be determined by interpolation between sampling values furnished by the demultiplexer at time intervals T.sub.so =1/f.sub.so.
Finally, it is necessary to provide in each data signal demodulator a pulse shaping filter for optimum suppression of the noise in the transmission path. This pulse shaping filter, which may be a Nyquist filter, must be optimally adapted to the transmitted signal.
In the prior art circuit of FIG. 3, the bandwidth limitation for each channel is obtained by the use of two filters 29 and 31 connected to the output of a demultiplexer 23. The outputs of the filters are added and subtracted at 33. Thus, the original pair of output signals 30a and 30b from demultiplexer 23 representing a complex-valued signal is temporarily reduced to one signal, a real-valued signal, before it is again split into a pair of phase displaced signals in the subsequent synchronous demodulator circuit.
The output signal of 33 is therefore multiplied with cos(2.pi.k(f.sub.m /f.sub.so) at 35, to shift the center frequency f.sub.m to zero, and then fed into the interpolation filter 43, where the timing adjustment is performed, and then fed into the pulse shaping filter 51 to yield the in-phase component which is fed into the decision circuitry 47.
In parallel the output signal of 33 is multiplied with sin(2.pi.k(f.sub.m /f.sub.so) at 36, then fed in the interpolation filter 46 and then fed into the pulse shaping filter 54, to yield the quadrature component which is fed into the decision circuit 47.
The decision circuit 47 decides which data was transmitted and can provide control signals for the timing adjustment 48 and for the center frequency 67.
The decision circuit itself is known in the art and not part of the present invention. Also the way in which the control signals 48 and 67 are generated is known in the art and not part of the present invention. Examples may be found in the publication John G. Proakis: Digital Communications, McGraw Hill, 1983, (FIG. 4.2.34 for control signal 67 and FIG. 4.2.41 for control signal 48).
Referring to FIG. 5a, there is shown the spectrum S.sub.DEMUX as a function of frequency f of the complex-valued output signal of the frequency division demultiplexer 23 of FIG. 3. In FIG. 5a, H.sub.BB is the frequency magnitude response of the filters 29, 31 with complex coefficients, from which only the real part of the output signal is further processed. In FIG. 5b, S.sub.1 is the spectrum of the real-valued output signal of the filters 29, 37, and, in FIG. 5c, S.sub.2 designates the spectrum of the complex-valued input signal of the interpolation filters IPF in the QAM demodulator. The interpolation filters have real coefficients, and are used twice for the real and imaginary parts of the time domain function corresponding to S.sub.2.
It is seen from FIG. 5a, that only the desired signal spectrum (solid line in FIG. 5a) can pass the band limiting filter. Undesired signals within other frequency ranges are suppressed (dashed lines). In FIG. 5c additionally to the signal spectrum (solid lines) the transfer of function of the interpolation filters is shown (dashed line).